1. Field of the Invention
The present invention relates to an insulated wire, especially to a resin composition for formed insulator, a foamed resin using the resin composition and an electric wire insulated with the foamed resin.
2. Description of the Related Art
With the recent progress of information and communication networks, data transmission cables used between apparatuses must cope with high speed and large capacity, and excellent transmission characteristics at high frequency are required. For realizing these requirements, an increasing number of data transmission cables adopt a method called “differential transmission” in which positive and negative voltages are applied to a two-core cable. This differential transmission method has a feature of high resistance against an extrinsic noise.
On the other hand, the method has a restriction of strictly controlling a signal transmission time difference (delay time difference: skew) between the two cores of the cable. This is to prevent communication errors that may occur in the receiving-side apparatus as the result of the occurrence of time difference between signals transmitted from a plurality of core wires.
Skew is a delay time difference between individual electric wires and significantly relates to a dielectric constant of the insulator of the electric wire. And a high-speed transmission cable requires an insulator having a low dielectric constant and thereby a high foaming degree. As the foaming degree of insulator is higher, the dielectric constant thereof becomes lower. Therefore, the foaming degree of the insulator is one of the most important factors.
To conduct differential transmission, the foaming degree of insulator must be uniform along the longitudinal direction thereof. In addition, to suppress fluctuation of the foaming degree, it is effective to make bubbles fine. As each of bubbles is smaller, the number of bubbles becomes larger in the constant foaming degree, thereby decreasing the fluctuation of foaming degree.
On the other hand, an insulator having a high foaming degree generally has a small amount of resin, causing problems in which mechanical strength is insufficient and deformation and buckling easily occur. In order to prevent these problems, there is a method to reinforce the foamed insulators by means of a cable jacket and the like, though, the optimal method to maintain stable performance of the foamed insulators is to make bubbles fine, thereby dispersing load and stress.
As explained above, an ideal cable for high-speed transmission adopting differential transmission is a cable which has a large number of fine and uniform bubbles and has no (least) fluctuation of the foaming degree throughout the entire length.
Generally, there are two foaming methods: One is a method that uses a chemical foaming agent (so called chemical foaming); and the other is a method in which gas is infused into molten resin in an extruder and foaming is executed due to a pressure difference between an inside and outside of a die of the extruder (so called physical foaming).
The chemical foaming method is advantageous since it is easy to obtain an insulator having a foaming degree that does not fluctuate much. However, there are problems in that it is difficult to achieve a high foaming degree and the dielectric constant of the insulator becomes large in relation to the degree of foaming because the dielectric constant of foaming agent residue is prone to become large. For this reason, foamed insulators manufactured by the physical foaming method are mostly used for the cables used for high-speed differential transmission.
In the physical foaming method, in order to maintain a certain degree of foaming while making the bubbles fine, a large number of bubbles need to be generated, and it is important to select a foam nucleating agent. Generally-used foam nucleating agents are inorganic particles, such as clay, silica and the like, high-melting point polymers, such as PTFE (polytetrafluoroethylene) powder and the like, and organic chemical foaming agents (azodicarbonamide (ADCA), 4,4′-oxybis(benzensulfonylhydrazide) (OBSH), and the like). Although optimal composition and shape of the foam nucleating agent differ according to the base resin and processing conditions, basically, it is well-known that the number of generated bubbles increases with becoming small the particles since the number of added particles significantly increases even though the amount of addition is the same.
However, a nucleating agent of fine particles easily agglomerates and it is very difficult to uniformly disperse the agent in the resin. That is, when fine particles are simply added to resin, agglomeration of the fine particles occurs, resulting in fluctuation of the foaming properties and, in the worst case, causing an adverse effect on the properties of the resin composition.
Also, there has been another problem with adding large amounts of nucleating agent. Essentially, a nucleating agent is a foreign substance, and because the dielectric constant of most foam nucleating agents currently being in practical use is larger than that of the matrix polymer, adding large amounts of nucleating agent adversely affects dielectric characteristics of resin composition, resulting in impairing the advantage of the foam.
In order to overcome the flocculation problem, JP-A 2005-271504 discloses a manufacturing method for a foamed resin article without using a nucleating agent. According to JP-A 2005-271504, a pre-molded resin article made of a resin (A) and a resin (B) that are non-compatible with each other and have mutually different solubility and diffusion coefficient of carbon dioxide and the mass ratio of 1/99 to 99/1 for (A)/(B), is impregnated with the carbon dioxide at temperatures below the melting points of the resins (A) and (B) and subsequently is foamed, thereby being provided a foamed resin article.